Surface acoustic wave devices, comprising a piezoelectric substrate upon which are deposited various configurations of conductive transducers, have found wide-spread application in the processing of electronic signals. Specifically, many television receivers employ such devices as a filter in the intermediate frequency section of the receiver.
An example of such a surface wave acoustic (SAW) filter is described in U.S. Pat. No. 4,146,851, "Acoustic Surface Wave Device", assigned to the assignee of the subject invention and hereby incorporated by reference. The filter described therein includes a substantially rectangular substrate that may be constructed from, for example, lithium niobate. At a lower corner of one end of the substrate is deposited an input transducer in the form of a series of interdigitated conductive elements. At the upper corner of the opposite end of the substrate is deposited a similarly arranged output tranducer, also including a series of interdigitated conductive elements. Also included in the filter are a multi-strip coupler and a reflector. The coupler causes signals launched by the transducer to be directed in quadrature to both the output transducer and the reflector. The phase relationship between the signals directed to the output transducer and the reflector results in substantial cancellation of "triple-transit" signals as thoroughly described in the cited patent. This is a highly desirable effect because the triple-transit signals result in a ripple and other undesirable anomalies in the phase and frequency responses of filter.
The above explication was intended to convey some of the complexities inherent in the fabrication of a SAW filter and indicate the substrate surface area required. A somewhat differently configured SAW filter is described in U.S. Pat. No. 3,872,410. One of the salient features of that device is that the piezoelectric substrate is in the form of a parallelogram characterized by an acute angle in the range of 20 to 25 degrees. The parallelogram configuration has been found to reduce the back reflections generated by the piezoelectric material. (See Col. 4, lines 44 to 52 of that patent).
The subject invention is directed to an improved substrate configuration that not only attenuates the level of reflected signals but also significantly reduces the amount of substrate material required. The reduction in substrate surface area results in a substantial reduction in the total cost of the device, largely because of the expense of the niobium component of, for example, a lithium niobate substrate.